Reservoir simulation often requires the numerical solution of the equations that describe the physics governing the complex behaviors of multi-component, multiphase fluid flow in natural porous media in the reservoir and other types of fluid flow elsewhere in the production system. The complexity of the physics that govern reservoir fluid flow leads to systems of coupled nonlinear partial differential equations that are not amenable to conventional analytical methods. As a result, numerical solution techniques are necessary.
A variety of mathematical models, formulations, discretization methods, and solution strategies have been developed and are associated with a grid imposed upon an area of interest in a geological formation. Reservoir simulation can be used to predict production rates from reservoirs and can be used to determine appropriate improvements, such as facility changes or drilling additional wells, that can be implemented to improve production.
A grid imposed upon an area of interest in a model of a geological formation may be structured or unstructured. Such grids are comprised of cells, each cell having one or more unknown properties, but with all the cells in the grid having one common unknown variable, generally pressure. Other unknown properties may include, but are not limited to, fluid properties such as water saturation or temperature for example, or to “rock properties,” such as permeability or porosity to name a few.
Grid generation may be constrained by the presence of geological faults, such that the generation of cells is driven by fault boundaries, with the fault boundaries defining the surfaces of adjacent cells. Cells may also be generated with non-cuboid geometries and varying volumes. Particularly with complex and/or highly-faulted geological formations, such “cut-cell” grids provide substantially greater accuracy over grids that enforce a regular array of identically sized cells.
On the other hand, algorithms that generate cut-cell grids may result in the creation of numerous arbitrarily small cells. It has been found, however, that the small cells can significantly impact performance, even with parallel simulators, and in some instances, may cause simulation errors.
Therefore, there is a need in the art for a manner of effectively and efficiently removing small and/or unwanted cells from a grid used for reservoir simulation.